At present, Nd—Fe—B (neodymium-iron-boron) magnets are mainly used as permanent magnets for applications where high magnetic force (maximum energy product) is required. However, Sm—Fe—N magnets are known as magnets which are superior in property to the Nd—Fe—B magnets (Patent Document 1 and Non-Patent Document 1). Sm—Fe—N magnets have the merits of being comparable in saturation magnetic polarization to the Nd—Fe—B magnets and higher in anisotropic magnetic field and Curie temperature than the Nd—Fe—B magnets and being less apt to oxidize and rust.
In general, powders for use as raw materials for magnets are classified by magnetism into isotropic magnet powders and anisotropic magnet powders. The term “isotropic magnet powder” means a powder in which each of the alloy powder particles is configured of a large number of fine crystal grains and the directions of easy magnetization of the individual crystal grains are random. Meanwhile, the term “anisotropic magnetic powder” means a powder in which each of the alloy powder particles is a single crystal or in which each of the alloy powder particles is configured of a large number of crystal grains and the directions of easy magnetization of the individual crystal grains in each particle have been oriented in a specific direction. The Sm—Fe—N alloy powders mainly include: isotropic magnet powders in which the main phase thereof has a hexagonal crystal structure that is metastable and is called the TbCu7 type and which is obtained, for example, by a melt-quench method; and anisotropic magnet powders in which the main phase thereof has a rhombohedral crystal structure called the Th2Zn17 type and is a stable phase.
The crystals which constitute Sm—Fe—N magnets decompose upon heating to a temperature exceeding about 500° C. Because of this, Sm—Fe—N magnets cannot be produced as sintered magnets, for which heating to a temperature around 1,000° C. is necessary during the production, and are used as bonded magnets. In general, a bonded magnet is produced by mixing a magnet powder and a binder and molding the resultant compound with a compression molding machine, injection molding machine, or the like. The bonded magnets hence are inferior in magnetic flux density to the sintered magnets by an amount corresponding to the presence of the binder and voids, but have a merit in that bonded magnets which are small or thin or have a complicated shape can be easily obtained. Furthermore, isotropic Sm—Fe—N bonded magnets produced from powders of TbCu7-type isotropic magnets are low in maximum energy product as compared with anisotropic Sm—Fe—N bonded magnets produced from powders of Th2Zn17-type anisotropic magnets, but have an advantage in that since there is no need of applying a magnetic field during the molding, the production efficiency is high and the freedom of designing magnetization patters is high. Owing to the merits of such isotropic bonded magnets and those merits of the Sm—Fe—N magnets (high anisotropic magnetic field, high Curie temperature, and low susceptibility to oxidation and rusting), isotropic Sm—Fe—N bonded magnets are used in, for example, automotive motors that are used in severe environments.
Patent Document 1: JP-A-2002-057017
Non-Patent Document 1: Ryo Omatsuzawa, Kimitoshi Murashige, and Takahiko Iriyama, “Structure and Magnetic Properties of SmFeN Prepared by Rapid-Quenching Method”, DENKI-SEIKO (Electric Furnace Steel), Daido Steel Co., Ltd., Vol. 73, No. 4, pp. 235-242, published in October, 2002